DOSBox is a free, open source and rather popular DOS emulator that can run on the Windows desktop right out-of-the-box with little knowledge of how the ---- Disk Operating System ---- works. A review of DOSBox has also been published.
Download DOSBox 0.74-3 (from Console Emulators)1.40 MB | For Windows 7, Windows 8 (64-bit, 32-bit) / Vista / XP | Free